Property and Casualty Premiums Written and Earned (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Ceded Credit Risk [Line Items]
Gross premiums written - direct	$ 280.6		$ 256.0		$ 933.3		$ 836.2
Gross premiums written - assumed	895.8	[1]	14.4	[1]	2,145.5	[1]	44.4	[1]
Ceded premiums written	(133.9)	[1]	(80.5)	[1]	(378.4)	[1]	(279.5)	[1]
Net premiums written	1,042.5		189.9		2,700.4		601.1
Gross premiums earned - direct	296.0		267.7		855.2		796.5
Gross premiums earned - assumed	943.3	[1]	14.0	[1]	2,134.3	[1]	30.3	[1]
Ceded premiums earned	(146.5)	[1]	(91.5)	[1]	(366.6)	[1]	(271.7)	[1]
Net premiums earned	$ 1,092.8		$ 190.2		$ 2,622.9		$ 555.1

[1] Amounts for the nine months ended September 30, 2012 include Transatlantic amounts from the Acquisition Date through September 30, 2012. See Note 2 for additional information on Transatlantic.